If the allele is sex-linked rec., then the boy's genotype is XaY.
His Xa came from his mother, who doesn't show the trait.
So his mother must be XAXa. So far, so good.
The father must be XAY, since he does not show the trait.
Then, can we get this daughter from those parents? The daughter has
at least one XA since she does not show the trait.
That XA could have come from either parent. And the daughter's
other X chromosome could bear either allele. Therefore,
the allele of interest here could be sex-linked recessive.
We can't find anything in the given information to rule that out.
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